A method and apparatus for processing a digital image to be displayed on a display panel illuminated with a backlight module. When the output signal from the oxygen sensor is outside the predetermined range, it is judged based on the output signal whether an actual air-fuel ratio is richer or leaner than the target air-fuel ratio, to compute the air-fuel ratio control signal based on the judgment result. The light beam is directed outward by means of a reflection free portion of the mirror glass, so that light passing through is directed towards the driver by means of the deflection optics.